1. Field of Invention
This invention relates to the field of analytical mass spectrometry. Specifically it introduces a new apparatus and methods for the identification and quantification of target compounds in a mixture by means of accepting an ion beam composed of either a single mass or ion clusters from an ion source, and further analyzing the ion beam by a controlled collision in a collision cell operated with direct current (DC) potentials, followed by mass analysis of the resulting fragments or de-clustered ions. More specially, the invention relates to the method of utilizing the collision cell to fragment ions at either high or low collision energy. Furthermore, the invention relates to methods of ionizing neutral fragments inside the collision cell and; also reacting ionic components within the collision cell with externally introduced counter ions or electronsxe2x80x94then analyzing these newly formed ionic species.
2. Description of Prior Art
Collisional induced dissociation (CID) is employed in tandem mass spectrometry is elucidate structural information of gas-phase ions derived from the ionization of organic and inorganic ions. The use of mass spectrometry to select gas-phase ions (precursor ions) for CID and analysis of the resulting fragments (product ions), has been extensively described in various reviews (Hoffmann, E., xe2x80x9cTandem mass spectrometry: A Primer,xe2x80x9d J. Mass Spectrom. 31, pages 129-137,1996; Busch, K. L., Glish, G. L., McLuckey, S. A., xe2x80x9cMass Spectrometry/Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry,xe2x80x9d VCH Publishers: New York (1988), Yost, R. A., Fetterolf, D. D., Tandem mass spectrometry (MS/MS) instrumentation,xe2x80x9d Mass Spectrom. Rev. 2, pages 1-45, 1983).
The oldest mass spectrometric configuration for tandem mass spectrometry is the combination of magnetic mass (B) and an electrostatic energy (E) sectors. The energy sector produces an ion kinetic energy separation providing information on metastable or CID ions. If a xe2x80x98field-freexe2x80x99 area is interposed between sectors, high energy ions are injected into this xe2x80x98field-freexe2x80x99 collision area undergoing collisions with the background gases (such as, helium or argon). Due to the low gas pressure (10xe2x88x923 torr), and the high forward velocity of the ion, there are very few collisions with the background gas. A collision, if it occurs, primarily involves a transfer of energy to the electrons of the moleculexe2x80x94resulting in odd-electron product ions. Because there are few collisions, the MS/MS spectrum typically shows a prominent precursor ion and low ion abundance of product ions.
If sectors are configured with a quadrupole collision cell and a quadrupole mass analyzer, both high and low energy collisions are possible (such as EBqQ). For low energy collisions, precursor ions are first selected with magnetic and electrostatic sectors, decelerated, then injected into high pressure quadrupole collision cell where they undergo multiple collisions with the background gas, and then mass analyzed with the second quadrupole. This Leads to product ion mass spectra that are similar to product ion spectra as observed in triple quadrupole MS/MS instruments (see below).
Other configurations utilizing sector analyzers have been configured and commercialized. In one alternative a time-of-flight mass analyzer (TOF) is placed in tandem with the sector analyzers and a surface induced collision (SID) cell, with the TOF mass analyzer performing high-resolution mass analysis of the product ions resulting from high energy collisions from SID.
The use of 3-dimensional ion traps (3-D IT) for MSIMS analysis using low-energy collisions is typified by Syka et al. (U.S. Pat. No. 4,736,101), Bier et al (U.S. Pat. No. 5,420,425), and Scwartz et al (U.S. Pat. No. 5,572,002). MS/MS analysis with the use of the 3-D IT uses at least two distinct mass analysis steps. First, a desired m/z is isolated in the trap by ejecting undesired ions during an ion accumulation step. This is performed using one of several techniques, such as applying a DC potential to the ring electrode, applying selective RF (waveforms), or scanning the RF so the undesirable ions are pass through the trap and are not accumulated. After the undesired ions are ejected from the trap, fragments or product ions can be formed when the ions that have remained in the trap are excited by applying a RF potential causing the ions to resonate (referred to as resonance excitation) and experience multiple collisions (low-energy) with the background gas, usually helium, inside the ion trap. The RF voltage (and possibly DC) is then readjusted to contain these lower mass fragments. The second MS step is then performed by ejecting the fragment ions by using a mass selective instability scan, such as, manipulation of the radio frequency amplitude, RF frequency, supplemental AC field amplitude, supplemental AC field frequency, or a combination thereof to eject ions out of the trap and collection and detection by a electron multiplierxe2x80x94thus performing two mass spectrometry steps with one device (MS/MS in time). Additional steps of accumulation, ejection, and fragmentation can be performed leading to MS/MSn (MS/MS to the nth degree, n=1, 2, 3, . . . ).
Other configurations utilizing 3-D IT assemblies as MSIMS analyzers have been configured and commercialized. In one alternative a time-of-flight mass analyzer is placed in tandem with a 3-D IT, with the TOF mass analyzer performing high-resolution mass analysis of the product ions. In others, a 3-D IT or a 2-D linear IT (q) replaces the third quadrupole in a triple quad system (see below), allowing further fragmentation of the product ions, leading to MS/MSn (see Bier et al., U.S. Pat. No. 5,420,425 for a 2-D linear IT). Recently a 2-D linear IT has been combined with a fourier transform mass spectrometer (Qq-FTMS), resulting in high-resolution mass analysis of the product ions similar to 3-D IT-TOF and the Q-TOF (see below).
Enke et al (U.S. Pat. No. 4,234,791) have described a quadrupole mass spectrometer system-three quadrupoles in tandem, typically referred to as a triple quad (QqQ). The first quadrupole is operated in a mode where both RF and DC voltages are applied to the rods and a resolution is chosen (by choosing a ratio of RF/DC ratio) to select one ion mass (or mass range) from the first quadrupole and then introducing it into a second quadrupole. The second quadrupole is operated with no DC voltage and at elevated pressures (millitorr range) relative to the first and third quadrupole, and only a relatively small RF voltage (usually ⅓-xc2xd of the Rf of the first quadrupole) is applied to the rods. In this mode, the second quadrupole acts as a xe2x80x98high pass mass filterxe2x80x99xe2x80x94rejecting the passage of all masses below a certain mass (commonly referred to as low mass cutoff) and passing all masses above this mass. Fragments, or product ions, can then be formed by passing (or injecting) the precursor ions from the first quadrupole into the second quadrupole at elevated pressures and colliding these ions with a neutral gas, such as argon, nitrogen, or just air in combination with a voltage difference (commonly referred to as in-lab collision energy, typically 10-100 ev) between the lens before the entrance to the first quadrupole and second quadrupole. The fragments in the second quadrupole are then passed to the third quadrupole.
The third quadrupole, operated in a similar manner to the first quadrupole, can pass one particular mass. This mode being commonly referred to SRM (selective reaction monitoring). Alternatively, the third quadrupole is scanned (varying the RF/DC ratio) and placing ions exiting the second quadrupole producing a mass spectrum of the collision fragment ions emerging from the second quadrupole. Other configurations utilizing quadrupole assemblies have been configured and commercialized. The third quadrupole has been replaced with a time-of-flight (TOF) mass analyzers, resulting in a Q-TOF instruments having the ability of producing high resolution mass spectra of product ion. Alternatively a 3-D IT or 2-D linear IT is used, resulting a Qq-IT or Qq-Linear IT instruments producing low resolution MS/MSn spectra.
Bergmann (U.S. No. Pat. 5,854,485), Cotter et al (U.S. Pat. No. 5,202,563), and Verentchikov et al. (U.S. Pat. No. 6,534,734) have described a time-of-flight (TOF) mass analyzers configured in tandem (TOF-TOF). Typically ions are injected into the first TOF, as they exit the first TOF precursor ions of a prescribed flight time are allowed to pass into a collision cell at low gas pressures (xcx9c10xe2x88x923 torr) all other ions, shorter and longer flight times, are deflected and not allowed to enter the collision cell. In the collision cell, the precursor ions undergo a small number of collisions resulting in high energy fragments. Any remaining precursor ions and product ions are then pulsed into the second TOF mass analyzerxe2x80x94resulting in high resolution mass spectra of product ions.
A commonly used alternative method of fragmenting molecules for mass spectrometry is referred to as xe2x80x9cin-beamxe2x80x9d fragmentation. This approach incorporates a high electric field in the free jet expansion into the sub-torr region of interfaces from atmospheric pressure ion sources. Usually this is the second stage of pumping in multistage interfaces. Early implementations of this approach are described by Fite (U.S. Pat. No. 4,209,696), French et al. (U.S. Pat. No. 4,121,099), and Kambara (U.S. Pat. No. 4,144,451).
In accordance with the present invention a higher pressure collision cell comprises both low-field and a high-field electrostatic regions, provided by direct current (DC) electrostatic potentials; the cell is pressurized with a inert or reactive gas to facilitate efficient collisional damping and collection of gas-phase ions.
The objective of the present invention is to increase the collection and reaction efficiency of gas-phase ions undergoing collisions or reactions with neutral background gases, reagent gases, or electrons; several objects and advantages of the present invention are:
(a) To provide a more versatile collision cell that can be operated both at relatively high [above 1 keV] or low [below 100 ev] collision energies.
(b) To provide a more highly pressurized collision cell (1-100 Torr) compared to conventional collision cells. The advantage of higher pressure allows more efficient and higher compression focusing of precursor and product ions compared to lower pressure cells where inertial components of motion make efficient focusing problematic.
(c) To provide a collision cell where the mean free path and electric field can be adjusted to yield a controlled precursor ion dissociation process by either single or multiple collisions.
(d) To provide collision cell where improved optics compression allows the use of smaller, lower conductance apertures compared to conventional optics.
(e) To provide a collision cell where the degree or extent of fragmentation can be controlled by means of adjusting the gas pressure, gas composition, electrostatic potentials, or a combination.
(f) To provide a collision cell where scattering losses are minimized compared to conventional lower pressure cells.
(g) To provide a collision cell where neutral fragmentation products can be re-ionized be a suitable chemical ionization reagent ion to provide capability of mass analyzing neutral fragment products.
(h) To provide a collision cell where multiply charged incident ions can efficiently capture electrons in order to undergo electron capture dissociation, with said dissociation products being efficiently collected, focused, and transmitted to a downstream mass analyzer.
Further objects and advantages are to provide a collision cell to use the collision cell to precisely control and determined the collision energy; and which can be used to introduce externally generated gas-phase ions into the collision cell to ionize gas-phase neutral components or neutral fragments (by means of chemical ionization), or react electrons with ionic components in the collision cell (as described in McLafferty, F. W., Horn, D. M. Breuker, K., Ying, G., Lewis, M. A., Cerda, B., Zubarev, R. A., Carpenter, B. K., xe2x80x9cElectron capture dissociation of gaseous multiply charged ions by fourier-transfrom ion cyclotron resonance,xe2x80x9d J. Am. Soc. Mass Spectom. 12, pages 245-249, 2001). Still further objects and advantages will become apparent from a consideration of the ensuing descriptions and drawings.